For deep space communication systems, the decision of whether or not to suppress the transmitted carrier has always been an issue. For certain missions that use high data rates, the available bandwidth might be a limiting factor. In such cases, it is preferred to use a completely suppressed carrier system that is more bandwidth efficient.
Radio science (RS) experiments currently rely on an unmodulated CW RF signal carrier for spectral purity and maximized signal-to-noise ratio. This requires missions to carefully schedule them away from periods of high-rate telemetry. In the era of optical communications, currently designed systems experience the same problem.
RS measurements are performed by using only a residual carrier scheme. When a carrier signal is passing through the atmosphere of a planet, RS information is extracted from the amplitude and phase variations of the received carrier. For certain critical missions, pure carrier transmission without data might be used for some duration of time to enhance the quality of received RS observations.
This work proposes using optical links for RS measurements. For RS, the optical signal can be unmodulated (known data) or modulated (with unknown data). In the proposed optical receiver, the modulation is considered to be a binary phase shift keying (BPSK) modulated laser, or an intensity-modulated optical signal such as pulse position modulation (PPM).
A data processing architecture was developed that will yield high-accuracy RS, or link science type of information on the ground from readily transmitted optical communication signals coming from space assets. This technique is intended to save power, bandwidth, and scheduling demands on the spacecraft. The approach is applicable to a broad suite of modulations (phase and/or intensity) and receiver types (coherent and/or non-coherent), thus providing an architectural improvement to present state-of-the-art communication systems utilized by NASA, as well as to future systems.
This method is an optical module to the existing optical communication receiver architecture. For optical links with intensity-modulated laser transmission or phase-modulated CW laser communications, the proposed optical receiver provides both data detection and signals required to extract RS data such as amplitude, phase, and frequency due to planetary atmospheric changes. The same information required for RS data can be extracted using differential methods of encoding. At the optical receiver, a local laser, a phase shifter, and an array of photon detectors are used.
This work was done by Dariush Divsalar, Bruce E. Moision, and Samuel J. Dolinar Jr. of Caltech for NASA’s Jet Propulsion Laboratory.